Priming compositions have undergone relatively gradual changes. In their early history, mercury fulminate was most commonly used. This material, however, was found to deteriorate too rapidly under tropical conditions. For a time, in the 1920's, lead thiocyanate/potassium chlorate formulations were used, but these proved detrimental to weapon barrels because they formed corrosive potassium chloride salts upon firing. Late in the 1930's a new class of primer mix, based upon lead styphnate, which is much more stable than mercury fulminate, was discovered and widely adopted. In fact, it is still widely in use today by rim-fire ammunition manufacturers. It, too, has its disadvantages in that it discharges lead and other heavy metals into the air upon firing. Consequently, the men of the art are looking for a suitable and improved substitute, even though they agree that lead styphnate has many advantages over most rim-fire primary explosives.
Center-fire and rim-fire primer mix requirements are considerably different because of the geometry of the metal parts in which they are used.
In a center-fire primer, the primer mix is positioned between a well defined anvil and the cup wall, and the mix can be processed so as to have a high material density. These two factors, positive cup and anvil surface interaction, and a high density of primer mix give a reliable ignition system.
In a rim-fire system, on the other hand, the anvil effect is not as positive because the rim of the case is only "pinched", and the high density of priming can not readily be achieved as it is spun into the rim in a wet "plastic" condition.
Therefore, rim-fire primer mixes must be intrinsically more sensitive to make up for the lack of material density and positive anvil effect.
Today, all commonly used rim-fire primer mix materials have three (3) main ingredients, namely: (a) a primary explosive, such as lead styphnate, (b) an oxidizing agent, such as barium nitrate, and (c) a fuel source such as antimony sulphide. Sensitizers, such as tetracene, and binders are also added to these above main ingredients. In the past ten years, however, many researchers have been seeking a non-toxic, or less toxic, primer composition. Many of the various patents which have issued show the significant drawbacks of the primer mixes heretofore utilized, as described hereinbelow. Many of these primer mixes were developed primarily for center-fire cartridges which, as described above, have a considerably different structure and mechanism for detonating the primary explosive.
One of the earlier patents issued to Krampen et al under U.S. Pat. No. 4,608,102, which uses diazodinitrophenol (DDNP) as the primary explosive and manganese dioxide as the oxidizer.
Another earlier patent issued to Hazel under U.S. Pat. No. 4,363,679. This invention utilizes a smokeless propellant, a titanium fuel and a zinc peroxide oxidizer.
U.S. Pat. No. 4,674,409, issued to Lopata, uses DDNP, tetracene, manganese dioxide and glass. It also uses a metal foil disk of compacted nitrocellulose which is positioned adjacent the primer mix in order to hold it in place. The disk requires an extra part and additional overhead and labor costs.
The Bjerke Pat. No. 4,963,201 also uses DDNP or potassium dinitrobenzofuroxanne as the primary explosive, nitrate ester as a fuel, and strontium nitrate as the oxidizer.
The inventors of the Bjerke et al patent, U.S. Pat. No. 5,216,199, compact some of the propellant against the primer mix in the belief that the primary explosive functions more effectively if made more dense. They utilize DDNP, tetracene, a suitable propellant, glass and strontium nitrate. Inclusion, herein, by reference thereto, is hereby made of the portion of said U.S. Pat. No. 5,216,199 which is entitled "Background of the Invention," because of its background discussions:
U.S. Patent No. 5,388,519, issued to Guindon et al, sets forth a helpful plurality of paragraphs in Columns 1 and 2 which describe many of the problems and their considerations in a primer having reduced toxicity. It, too, suggests the use of DDNP as the primary explosive along with a mechanical frictionator (glass or aluminum), a fuel, and an oxidizer selected from a group which includes strontium sulphate and strontium oxalate as well as cupric or ferric oxide. It points out that the presence of tetracene can cause thermal instability. It also lists a number of additional U.S. patents which are relevant to the development of a new and improved primer mix.
U.S. Pat. No. 4,675,059, issued to George C. Mei, discloses a non-toxic primer mix which contains DDNP, manganese dioxide as an oxidizer, tetracene and glass.
U.S. Pat. No. 4,963,201 issued to Bjerke et al, also suggests the use of DDNP or potassium dinitrobenzofuroxanne as a primary explosive, tetracene, a nitrate ester fuel, and strontium nitrate.
U.S. Pat. No. 5,167,736, issued to Mei et al, discloses a non-toxic primer which is principally comprised of DDNP and boron. It may also contain calcium carbonate or strontium nitrate as an oxidizer, a nitrate ester as a fuel, and tetracene as a secondary explosive.
The above patents and all others which utilize DDNP as the primary explosive for primer mixes are probably more effective for center-fire cartridges than for rim-fire cartridges, because the anvil construction of the primer body of the center-fire cartridges compensates for the lack of adequate sensitivity found in DDNP. The lead-free center-fire mixes have been fairly successful, as such, but when utilized as rim-fire primer mixes, they have not been sufficiently reliable to permit their use for regular commercial ammunition.
As indicated above, lead styphnate is in common use today as a primary explosive. Also barium nitrate is a heavy metal oxidizer which is used today by almost all rim-fire primer manufacturers to cause the fuel to burn more effectively. Because of the heavy metal presence in its composition in barium nitrate, as well as in antimony sulphide, the United States Federal Bureau of Investigation is seeking a less toxic composition.
It has long been known that cupric azide is a sensitive and powerful producer of flame, and is very brisant. However, unlike lead azide, it has not previously been used in primer mixes, especially in non-toxic mixes. It has undoubtedly been tried, but has never been found to be acceptable for regular commercial ammunition manufacture.
Cupric azide is so highly sensitive that heretofore it has been generally ruled out of consideration for use as a safe primary explosive for rim-fire primer compositions. We have discovered, however, that it can be utilized safely, if mixed with the proper materials and in the right proportions, as described hereinafter. Its use makes it possible to eliminate the presence of lead styphnate, and thereby obviate the adverse consequences of vapors of lead which are presently associated with the firing of primer mixes which are based upon the use of lead styphnate as the primary explosive.